This invention relates to a martensitic stainless steel that is excellent in corrosion resistance and stress corrosion cracking resistance and to a method of heat treatment of the steel. More particularly it relates to a high-strength steel that has high corrosion resistance and cracking resistance in an environments containing wet carbon dioxide and wet hydrogen sulfide, for example, in well drilling for and transportation and storage of petroleum and natural gas, and to a method of heat treatment of the steel.
Petroleum and natural gas produced recently contain much wet carbon dioxide in increasingly many cases. It is well known that carbon steels and low-alloy steels corrode greatly in those environments with carbon dioxide. For this reason, corrosion inhibitors have so far been added to prevent the corrosion of OCTG (Oil Country Tubular Goods; e.g. casings and tubings) used for production and of line pipes used for transportation. However, corrosion inhibitors often lose their effects at high temperature and besides the cost required for the addition and recovery of corrosion inhibitors is immense in off-shore oil wells and submarine pipelines; therefore, corrosion inhibitors cannot be used in many cases. For this reason, the need of corrosion-resistant materials that do not require the addition of corrosion inhibitors has recently become very great.
The application of stainless steels with good corrosion resistance was first examined as corrosion-resistant materials for petroleum and natural gas containing much carbon dioxide. For example, as in L. J. Klein, Corrosion/'84, Paper No. 211, martensitic stainless steels containing 12 to 13% chromium, such as AISI type 410 and 420 steels, begin to be used widely as steels that have high strength and are produced at relatively low costs. These steels, however, have the disadvantage that they do not show satisfactory corrosion resistance and exhibit large corrosion rates at high temperatures of more than 130.degree. C., for example, or at high concentrations of Cl.sup.- ions even in an environment with wet carbon dioxide. These steels have another disadvantage that when petroleum and natural gas contain hydrogen sulfide, their corrosion resistance deteriorates greatly, thus causing general corrosion and localized corrosion, and further even stress corrosion cracking (in this case, sulfide stress cracking, hereinafter referred to as SSC). Therefore, the use of the above martensitic stainless steels has so far been limited to a case where the environment contains an ultratrace amount of H.sub.2 S gas, for example, the partial pressure of H.sub.2 S gas is not more than 0.001 atm or the environment does not contain H.sub.2 S gas in the least.
The steels described in Japanese Patent Unexamined Publications 60-174859 and 62-54063, for example, have been proposed as martensitic stainless steels in which the resistance to the cracking by hydrogen sulfide is improved. However, the cracking by hydrogen sulfide is not completely prevented in these steels. In addition, these steels have the disadvantage that the cost is high because nickel, which is an expensive alloying element, is used in large quantities.